       Re: Occurrences in Mathematica

• To: mathgroup at smc.vnet.net
• Subject: [mg106374] Re: Occurrences in Mathematica
• From: Peter Pein <petsie at dordos.net>
• Date: Mon, 11 Jan 2010 05:28:16 -0500 (EST)
• References: <hic33o\$59f\$1@smc.vnet.net>

```George schrieb:
> Hi,
>
> I have a very big list of elements and I need Mathematica to give me
> only those elements which occure only once...
>
> Could anybody advise me please how to write the code for that?
>
> Thks much
> George
>

Hi George,

calculating some data:

SeedRandom;
theList=Insert[RandomChoice[#/Tr[#]&[1/#^4]->#,{10^6}]&[Range],{a,b,a,c},123456];

to preserve the order of occurrance, I would use:

Reap[
NestWhile[
(If[FreeQ[##1],
Sow[#2];#1,
DeleteCases[##1]]&)@@{Rest[#1],First[#1]}&,
theList,
#1=!={}&]
][[2,1]]

which gives: {26,29,35,{a,b,a,c},40,60,28,34,33,31,32}

If you want sorted results, use the simple (and fast)

Cases[Split[Sort[theList]],{x_}:>x]

---> {26,28,29,31,32,33,34,35,40,60,{a,b,a,c}}

HTH,
Peter

```

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